Generally, copper-nickel alloys exhibit excellent properties in corrosion resistance, ductility, processability, and high temperature characteristics by changing a ratio of copper and nickel, and also has characteristic properties in electrical resistivity, coefficient of heat resistance, thermal electromotive force, coefficient of thermal expansion, and the like. Thus, studies have hitherto been conducted to obtain such properties of copper-nickel alloys by electroplating. As conventionally attempted copper-nickel alloy electroplating baths, a large variety of baths have been studied, including a cyanide bath, a citric acid bath, an acetic acid bath, a tartaric acid bath, a thiosulfuric acid bath, an ammonia bath, and a pyrophosphoric acid bath; however, none of these baths have been put into practical use. The reasons why the copper-nickel alloy electroplating has not practically been used include: (i) copper and nickel differ from each other in deposition potential by approximately 0.6 V, so that copper is preferentially deposited; (ii) the plating bath is unstable, so that insoluble compounds such as metal hydroxides are generated; (iii) the plating composition varies due to energization, so that coating having a uniform composition cannot be stably obtained; (iv) the service life of the liquid is short; and the like.